A Comparison of Pseudomorphic Bcc Phase Stability in Zr/Nb and Ti/Nb Thin Film Multilayers

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M.K. Miller Metals and Ceramic Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831

H.L. Fraser Materials Science and Engineering Department, The Ohio State University, Columbus, Ohio 43210 (Received 9 May 2003; accepted 8 October 2003)

A series of Nb-rich Zr/Nb and Ti/Nb multilayers were sputter deposited. Upon a reduction in thickness, a pseudomorphic bcc phase was stabilized in the Zr and Ti layers. X-ray and electron diffraction techniques were used to confirm these phase transformations. The change in phase stability was modeled by the competition between volumetric and interfacial components of the total free energy of a unit bilayer representing the multilayer. An outcome of this model is the ability to plot phase stability diagrams for multilayers, referred to as biphase diagrams, as a function of bilayer thickness and volume fraction. A comparison of the phase stability boundary between hcp/bcc and bcc/bcc for these two systems has shown that the bcc Ti’s pseudomorphic phase stabilization is maintained for a much larger layer thickness as compared to Zr. Atom probe compositional profiles of the Ti/Nb multilayers have indicated that the Nb layers interdiffused into the Ti layers thus helping to facilitate the bcc Ti phase stability in the Ti/Nb multilayers.

I. INTRODUCTION

When thin films grow epitaxially on a substrate, crystallographic phases that are not observed in the standard state of 1 atm and 298 K may result in the film during the early stages of growth. This is referred to as pseudomorphic growth. Formation of pseudomorphic phases has been reported for single thin films on substrates1–3 as well as in individual layers in multilayered thin films.4–6 Recently, Dregia et al.7 have reported a model, based on classical thermodynamics, that can predict phase stability of pseudomorphic phases in thin film multilayers. In this model, pseudomorphic phases are stabilized by the competition between the volumetric and interfacial components of the total free energy. At a critical transition thickness, the decrease in the interfacial free energy component can more than compensate for the increase in the volumetric free energy component, and a pseudomorphic phase within one (or both) layers of the multilayer can be stabilized. Dregia et al.7 have modeled this change in phase stability in multilayers by considering a unit bilayer representing the A/B multilayered stack. This unit

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Present address: Department of Metallurgical and Materials Engineering, University of Alabama, Tuscaloosa, AL 35487. J. Mater. Res., Vol. 19, No. 3, Mar 2004

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bilayer consists of a layer of A, a layer of B, and two A/B interfaces. The two independent degrees of freedom in the unit bilayer are the bilayer thickness, ␭, which is equivalent to the layer thickness of A plus B, and the volume fraction, ƒi, where i is either A or B. The free energy difference between the pseudomorphic and bulk equilibrium states of the multilayer is then given as ⌬G = 2⌬ ␥A + 关⌬GA

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